I have a S&W model 65 which was a range rental gun and it's barrel was replaced with that of a model 64. It is shooting about 4" high at 15 yards. Would it be possible to build up the front sight with silver solder and then file it until I get the height I want? It would not be permanent as eventually I would try to have it replaced when I determined the optimal height for the loads I will use with it. I have done this before but never with a stainless gun.

Define 'silver solder' !! There is 'soft solder' which is a 95Sn5Ag solder which melts at about 400 F and there is a 'hard silver solder ' which melts at 1100 F or more . the hard solder should be called a' braze'.
The soft solder may not have the strength but shouldn't cause other problems. The hard solder has the strength but the heat applied may damage the barrel from warping.
Welding ,TIG, and taking care to keep the barrel cool would probably be the best choice but knowing the barrel's alloy would be important.

You really can't use silver braze (solder) to build up metal.
The solder tends to melt and run, which it's intended to do, so it tends to flow off the top of the sight.

You possibly could do it using one of the silver brazes that's made to fill gaps. This doesn't run like the thinner types do. Another problem with brazes is that they tend to form a "bubble" on flat surfaces.
In other words, the middle of the solder will be much thicker than the edges.
This might make it difficult to get the entire surface built up enough so that when it's filed flat it's still thick enough:

A much better option would be to have a GOOD welder use Heli-arc or Tig welding to build up the top using a stainless rod.
This would work better, allow much less heat spread through the barrel, and would be real stainless steel on the stainless sight.
It would also be tougher and a closer color match.

As a temporary, not durable expedient, you could use an epoxy metal to build up the sight. This would last a few shots, especially if you drilled a couple of shallow holes on top of the sight to give the epoxy a "lock" on the sight.

Finally, what I've done before is to make a "try sight" from heavy wire about the thickness of the sight.
Bend it into an "L" shape and use heavy tape to tape it to the barrel in front of or just behind the sight.
Paint the original sight with white paint, and paint the try sight with black so you can see it better.
Use that as a sight and file it down until you're targeted.
Then measure the try sight for height.

...The hard solder has the strength but the heat applied may damage the barrel from warping...

If I am not mistaken, Remington bolt handles are silver soldered on to the bolt body. Also, silver solder (high temp.), has been used to fasten the front sights on many pistols including Ruger Super Blackhawks.

It can't be that outdated a term, when you order solder to make tools, be it tape, sheet, or coil, it is ordered as silver solder. I suspect the other terms came into affect so plumbers and refrigeration techs would know which application was called for with out confusion.

File out a slip over sight from metal and attach it with low temp silver solder.
Heck, you could even use black colored Acetal (Delrin) plastic and glue it on.
It's available from Enco (www.use-enco.com)
You could also use aluminum and a Dremel tool to open up a slot from just the one side. Shape it and use aluminum black on it. You would have to be careful with the Dremel in making the slot, but it would allow a sold front to the sight.
Now you have a few other ideas. Pick from mine or one of the others suggested and go for it...Have fun.
It might take a little work, but if it's just temp until you get another barrel, it might be the best option. The try sight that was mentioned in an earlier post is a very good idea. Then you will know how high you need to build the sight. Leave a little extra for fine tuning on the top.
A piece of metal with a slot filed through the middle, but not all the way to the top would work. You can then thin out the sight at the top for proper width.
Kind of a Bubba fix, but it might be the easiest way to do the job.

Which ever type of silver solder you choose, be sure the parts fit as well as possible then sweat solder on both parts, clamp togather then apply heat until the solder melts and makes a good joint. You want as little space between the parts as possible and want to use as little solder as possible and still get the parts to hold solid.
You can heat the parts and let the solder wick into the joint, but I like to flux and then sweat the solder onto each part first.
If you do that, you will have a strong attachment.

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